1. Field of the Invention
This invention relates to an inverter device comprising a main circuit which includes a DC power supply, an inversion section and a current-detecting resistor, the inverter device detecting current on the basis of voltage between both ends of the resistor.
2. Description of the Related Art
Motor current needs to be detected precisely for the purpose of feedback control or overcurrent protection control when an inverter device is used to drive an electric motor, for example. Japanese Patent Publication Nos. JP-02197295A and 09229972A disclose current detectors used for the aforementioned purpose respectively. Each disclosed current detector includes a shunt resistor (current-detecting resistor) connected to a DC power supply line between a DC power supply and an inversion section or connected between lower arm side switching elements of the inversion section and the DC power supply line. Voltage developed across both ends of the shunt resistor is amplified by an operational amplifier or isolated amplifier thereafter to be supplied to an AID converter. A processor is provided for obtaining a motor current from a detected current value of the shunt resistor and a PWM drive signal generated by itself.
In the foregoing arrangement, the operational amplifier or isolated amplifier is used as a circuit for processing the voltage across both ends of the shunt resistor. The reason for the use of the amplifier is as follows. The voltage across both ends of the shunt resistor is positive or negative depending upon a direction of current. Accordingly, the voltage cannot directly be supplied to the processor including the A/D converter or A/D conversion function energized from a single power supply. Furthermore, voltage drop is small in the shunt resistor, which is convenient or advantageous when the voltage is amplified so as to be ranged in an input voltage of the A/D converter or processor.
However, using an operational amplifier as the current detector complicates the circuit arrangement and requires both positive and negative power supplies. As a result, a scale and cost of the circuit are increased. Furthermore, the operational amplifier has a delay in operation, which results in a delay in current detection. Consequently, a high-speed and high-precision operational amplifier which is accordingly high in the cost is required in order that a necessary precision in the current detection may be ensured.
Therefore, an object of the present invention is to provide an inverter device with a current detector which is low-cost without use of an operational amplifier and can detect current with high precision.
The present invention provides an inverter device comprising a DC power supply, an inversion section connected via a DC power supply line to the DC power supply to be supplied with DC voltage from the DC, power supply, thereby delivering AC voltage, a current-detecting resistor connected to the DC power supply line and having both ends, and a current detector detecting current on the basis of voltage developed across said both ends of the current-detecting resistor, the current detector including a voltage-dividing resistor connected between a level-shift power supply line having a predetermined level-shift voltage and one of the ends of the current-detecting resistor, the voltage-dividing resistor having a voltage-dividing point, the current detector further including an A/D converter having a pair of reference potential terminals and an input terminal, one of the reference potential terminals being connected to the other end of the current-detecting resistor, the input terminal being connected to the voltage-dividing point of the voltage-dividing resistor.
Positive or negative current flows into the current-detecting resistor depending upon the state of the DC power supply, inversion section, or a load (an electric motor, for example) connected to the inversion section. When said one reference potential terminal of the A/D converter (said other end of the current-detecting resistor) is at a reference potential, the potential of said one end of the current-detecting resistor changes between positive and negative values in proportion to the aforementioned current. In thee above-described current detector, said one end of the current-detecting resistor is pulled up through the voltage-dividing resistor to the level-shift power supply line. Accordingly, the potential at the voltage-dividing point of the voltage-dividing resistor is shifted in the positive or negative direction depending upon the polarity (positive or negative) of the level-shift voltage. Consequently, positive or negative unipolar voltage in proportion to the aforementioned voltage is applied to the input terminal of the A/D converter.
The foregoing inverter device excludes an operational amplifier and a circuit arrangement accompanying the operational amplifier. Consequently, since the circuit arrangement of the current detector is simplified, a cost reduction can be achieved. Furthermore, since no operational amplifier is used, the inverter device call prevent a delay in the detection timing resulting from a delay in the operation of the operational amplifier. Additionally, the current can precisely be detected when a high-precision current-detecting resistor and a high-precision voltage-dividing resistor are used in the current detector.